Flora alpina: On seeds, spores, and double identity
The best-kept botanical evolutionary secret
~ Note: This is (yet another) introductory plant evolutionary biology post before we explore the plant families of the Alps. To get further oriented, check out my other intro posts on the Tree of Life and plant families. ~
What if I told you that plants have sex via secret identities?
To properly tell the story of the Plant Tree of Life (see my introduction here), we have to crack open some of plants’ deepest family lore. As one of the fiercest drivers of evolution, reproduction—the ways plants pacakge their genes and make their trysts—is also a deep Tree of Life branch-splitter. There are some very different ways to go about plant sex, and these methods define whole clades.1 But these histories are also deeply, unexpectedly shared. And herein lies our story.
First, let's talk about spores.
Our default image for a plant includes the seed: a dense, tidy package that can be poked into the ground, germinate, and grow into a plant that looks like the one that made the seed. In fact, this reproductive packaging is a more recent evolutionary innovation than overall planthood itself. Only one major branch of the Plant Tree of Life produces seeds: Spermatophyta, i.e. seed plants—ones bearing cones and fruits, i.e. increasingly fancy ways to house and disperse seeds (see Figure 1).
The ancestors of seed plants, however, had no need for seeds. Neither do today’s bryophytes (mosses, liverworts, hornworts), ferns, or lycophytes. They have spores.
It takes two (generations)
Spores and sex
Without seeds, plant reproduction is a more scattered affair, happening in two stages. Let’s follow along with a fern. Picture those (somewhat creepy) rows of brown clumps you may have noticed on the underside of a fern frond: this is where spores are made. In Stage One, these tiny, single-celled spores clustered on those spore-bearing fronds are the main vehicle of genes. They will waft off on air or water currents to find somewhere new to grow.
How are spores different from seeds? Whereas seeds are the result of sexual reproduction, containing a full-fledged embryo, spores are asexual, the result of a splitting cell. A spore only has half of the original plant's chromosomes2, so it's not going to grow into a clone of its parent. It's not even going to look anything like it.
The spore, having chanced on a suitable (usually moist) spot, grows into a new, secret plant. For a fern, this is a very small, flat heart-shaped plant with sexual organs and not much else. Its purpose is to launch Stage Two by making gametes: the sperm or egg that will go on to fuse with its complement and sexually produce an embryo.
In vascular plants, that embryo will grow directly out of the little secret plant into a more developed plant, the one that we humans think of as the plant.3 The fern, the horsetail. But that visible identity is only half the picture.
This double identity in plants is called "alternation of generations." The plant that we notice (the fern with fronds) is called the sporophyte because it makes spores, and the little sexually active plant is called the gametophyte because it makes gametes. Remember these terms; they’re the key to the rest of the story.
The secret sexual identities of seed plants
(The part that really blows my mind)
Studying plant evolution is like learning a second language and realizing that most of the vocabulary you took for granted in your mother tongue has a secret, deeper, other life.4
Because guess what? The same process—alternation of generations—is also going on in seed plants. Seed plants make spores too, just like their ancestors. Trees? They're sporophytes.
What seed plants did to keep this secret was to shrink their gametophytes—the sexual identity—so small, they never leave the spore-bearing structure that made them. Pollen is in fact a tiny bomb-proof case for a living male gametophyte that grew from the spore trapped inside it. You may notice in the image below that pollen grains resemble spores themselves—but they’re a few cells and a generation beyond one.
Fruits and female cones of seed plants make ovules, which look like like unfertilized seeds. Inside each ovule, a few cells split into spores, and one becomes a female gametophyte. This microscopic mother plant has just enough cells to house an egg, which can then be fertilized by sperm from pollen5 to make the seed and its embryo, right there on the sporophyte plant.
That embryo grows into a new sporophyte. A pine tree, say. Before the tree can make seeds, it must in turn spawn its tiny internal children. The seeds will technically contain the tree’s grandchildren.
It's like plant sex Inception.
Why care about sporophytes and gametophytes?
Maybe you’re not convinced (or you’re just confused); maybe you wonder why a few extra cells inside a pollen grain or ovule are worth thinking about, or describing as a whole other individual inside its parent6. It’s true that it makes no real difference to our experience of a plant, especially a seed plant. A tree is a tree, a seed is a seed.
But to me, these ephemeral beings, the secret, spore-spawned gametophytes, are a stunning example of how plants can hack their evolutionary heritage, but are never entirely free of it.
And in fact, pollen grains, cones, and fruit owe much of their form and function to the infrastructure designed for bearing spores (since that is exactly what they’re doing).
Comparative anatomy
This parallel history becomes clearer when we make a closer comparison with the seedless branches of the Plant Tree of Life. This requires some more vocabulary for precision’s sake, so buckle up.
Spores are made in spore factories called sporangia, found on fertile leaves called sporophylls. Sporangia are what are clustered on the undersides of fern fronds (i.e., fern sporophylls). In some plants, like the horsetail family (Equisitaceae, a fern relative), those leaves are clustered at the tips of the stems, and we call them strobili (see horsetail image above).
What is a pinecone? It’s a strobilus. Each inner bract is technically a sporophyll, adapted for making pollen or seeds (via the gametophyte).
While most spore-bearing plants produce physically identical spores, others produce two sizes of spores. Some lycophytes, including quillworts (Isoetaceae), have sporangia that only make megaspores, and others that make microspores. The megaspores, in turn, make bigger, nutrient-stocked female megagametophytes, and the microspores make minimalist male microgametophytes. This is called heterospory (two different sizes of spores). In quillworts, neither type of gametophyte leaves their spore casing—just like seed plant gametophytes. That’s called endospory.
Quillworts aren’t the ancestors of seed plants, but seed plants’ ancestors did something similar: hetorospory and endospory were key waypoints on the evolutionary path to Spermatophyta.
So, what is pollen? A microgametophyte. The anthers that produce pollen in flowers are full of microsporangia. Pollen, and microgametophytes in general, are optimized to get around.
Ovules, in fruits and female cones, are megasporangia that make megagametophytes. Continually nurtured by the parent sporophyte, megagametophytes can increase the chance of offspring survival. This pushed the ancestors of seed plants to invest more and more in this strategy.
Proper seeds required a few more evolutionary steps, as did fruits. But that’s another story.
With this sexual evolutionary history out in the open, it will be easier to talk about our first few Alpine representatives of the Plant Tree of Life. Lycophytes, which I briefly mentioned here, are spore-bearing plants, and descendants of some of the oldest spore-bearing plant ancestors. See you next time with more on club mosses and quillworts!
P.S. Ok, I’m curious. If you made it this far, how well did you follow that introduction to the evolution of plant reproduction? It’s such a convoluted topic, and one I’ve always wanted to figure out how to communicate in an accessible way that doesn’t result in glazed-over eyes. This is my first formal attempt, so feedback is appreciated—via the poll below, or feel free to reply or comment with more specific reactions!
Clade is another word for branch/group.
I.e., a spore is haploid, not diploid like the parent. The splitting that generated the spore was meiosis, rather than mitosis, if you remember those terms from Biology. Thus, haploid: one set of chromosomes.
In mosses and other bryophytes, it’s the opposite; the sexual plant is the dominant stage, and the spore-bearing plant is the tiny one, which pops up out of a moss’s sexual organs.
There’s a reason linguistics uses a lot of the same methods and analogies as evolutionary phylogenetics.
The little guy inside the pollen can actually extend its cells to grow down inside the female structure of a plant to fertilize it; this is called the pollen tube.
And in fact some plant biologists would probably say it’s splitting hairs to call a seed plant’s gametophyte a separate individual. But the concept is useful when we’re talking about evolution.
You ask "If you made it this far, how well did you follow that introduction to the evolution of plant reproduction? " and my answer is that 50+ years after studying this stuff at college this is perhaps the best explanation I have come across. On the other hand, this is one of the reasons that I went down the animals biology route rather than the plant one. Thanks - it's good.
Heterospory! Makes complete sense. I never knew!